The invention relates generally to the field of marine geophysical surveying. More specifically, the invention relates to systems for determining stretch in geophysical streamers as a result of towing forces.
Marine geophysical surveying systems such as seismic acquisition systems and electromagnetic survey systems are used to acquire geophysical data from formations disposed below the bottom of a body of water, such as a lake or the ocean. Such geophysical data may indicate geophysical structures and/or identify formations likely to contain useful materials such as water, oil, or natural gas. Marine seismic surveying systems, for example, typically include a seismic survey vessel having onboard navigation, seismic energy source control, and geophysical data recording equipment. The seismic survey vessel is typically configured to tow one streamer or, more typically, a plurality of laterally spaced apart sensor streamers through the water. At selected times, the seismic energy source control equipment causes one or more seismic energy sources (which may be towed in the water by the seismic vessel or by another vessel) to actuate. Signals generated by various sensors on the one or more streamers in response to detected seismic energy are ultimately conducted to the recording equipment. A record with respect to time is made in the recording system of the signals generated by each sensor (or groups of such sensors). The recorded signals are later interpreted to infer the structure and composition of the formations below the bottom of the body of water. Corresponding components (transmitters and receivers) for inducing electromagnetic fields and detecting electromagnetic phenomena originating in the subsurface in response to such imparted fields may be used in marine electromagnetic geophysical survey systems.
The one or more sensor streamers are in the most general sense long cables that have geophysical sensors disposed at spaced apart positions along the length of the cables. Streamer strain members may function to convey the axial load of the streamer, thereby preventing loading of other, more sensitive components. A typical streamer can extend behind the geophysical survey vessel for several kilometers. Due to manufacturing, transportation, and operational logistics, streamers are typically made up of a number of segments, connected at axial ends. Because of the great length of the typical streamer, large frictional forces are generated as a result of towing the streamers in the water. Such frictional forces can impart substantial axial loading on the streamers, thereby elongating the streamers. For example, some streamers may stretch approximately 0.5-1% under full operational load. During electromagnetic surveys, this may result in an over-estimation of roughly the same magnitude in the electric field measurement.
Accurate survey results depend on accurate knowledge of the distance between the energy source and each of the sensors on each streamer. For example, some electromagnetic surveys require absolute uncertainty of the measured result to be below about 1% to be able to accurately determine the resistivity of detected anomalies. There exists a need for a method to compensate for streamer elongation in measurements made by sensors in towed marine geophysical streamers.